The field of the subject invention is the production of viscose from cellulose.
In the early processes for the production of viscose, cellulose is first steeped at about 20.degree.-50.degree. C. in 17 to 22 percent aqueous NaOH (unless otherwise specified, all percentages herein are by weight) resulting in alkali cellulose. NaOH concentrations of less than 17.0 percent have not been reported heretofore as being useful in this single steeping operation. In order to achieve a proper viscosity in the final viscose, the alkali cellulose is then depolymerized, normally accomplished by allowing the alkali cellulose to react with atmospheric oxygen or an oxidizing reagent such as hydrogen peroxide.
After depolymerization, the alkali cellulose is reacted with carbon disulfide (CS.sub.2) to produce a cellulose xanthate. If the NaOH concentration in the alkali cellulose is higher than about 16 percent, an unacceptable amount of CS.sub.2 is required to effectuate this xanthation step due to side reactions of the CS.sub.2 with the excess caustic. Therefore, it is well known in the art that prior to xanthation, the alkali cellulose can be re-steeped in an aqueous solution of about 10-12 percent NaOH to reduce the NaOH concentration in the alkali cellulose. This double steeping process, of course, results in increased manufacturing costs.
Following the xanthation step, the cellulose xanthate is mixed with a dilute aqueous NaOH solution in order to dissolve the xanthate, filtered to remove undissolved particles and foreign matter and extruded into an acid bath to regenerate the cellulose in the form of spinnable filaments in the case of rayon and film in the case of cellophane. The filtering step following the dissolving of the cellulose xanthate is a test of the success of the viscose preparation process in that if the original alkali cellulose does not contain at least about 12 percent NaOH, the cellulose xanthate made therefrom will not dissolve sufficiently and hence will clog the subsequent filtering process.
Recognizing the advantages of a low NaOH concentration single steep process, U.S. Pat. No. 2,614,102 issued on the application of Schlosser et al discloses forcing at least 12% aqueous NaOH through sheets of paper pulp at high pressures thus maintaining contact between the cellulose molecules and a constant concentration of NaOH. This process, however, requires a cumbersome mixing apparatus.